Heretofore, a motor for rotating a recording medium formed in a disc shape, at a predetermined rotational speed, is installed in a hard disc drive (referred to hereunder as HDD) mounted onto a terminal unit such as a fixed type personal computer, a notebook type portable personal computer, or the like (for example, refer to Japanese Patent No. 3184794).
As shown in FIG. 4, this motor is provided with a fluid dynamic bearing 71, which enables the recording medium to rotate freely. The fluid dynamic bearing 71 is provided with a cylindrical shaft body 73 whose cross section is approximately cruciform, a sleeve 75 having a shaft body insertion hole 75a whose cross section is approximately cruciform and which houses the shaft body 73 such that it can rotate freely, and liquid 77 such as oil, filling the clearance between the shaft body insertion hole 75a and the shaft body 73.
Furthermore, cores and coils are provided on the outer peripheral surface of the sleeve. A rotor formed substantially cylindrically and with a base is installed on the top end of the shaft body, and a ring shaped permanent magnet is fixed on the inner peripheral surface of the cylindrical wall part of the rotor, facing the cores and the coils. The cylindrical wall part of the rotor forms a return yoke for constraining the direction of the magnetic flux entering and leaving the inner peripheral surface of the permanent magnet located on the opposite side to the outer peripheral surface facing the cores and the coils.
However, in the conventional motor, since the cores 79 and the coils 81 are located on the inner peripheral surface 85b side of a permanent magnet 85, it is not possible to reduce the diameter of a rotor 83 to which the permanent magnet 85 is fixed. Therefore, there is a problem in that there is a limitation to miniaturization of the motor.